import type { MonoTypeOperatorFunction, ObservableInput } from '../types.js';
import { Observable, operate, from } from '@rxjs/observable';
import { noop } from '../util/noop.js';

/**
 * Returns an Observable that skips items emitted by the source Observable until a second Observable emits an item.
 *
 * The `skipUntil` operator causes the observable stream to skip the emission of values until the passed in observable
 * emits the first value. This can be particularly useful in combination with user interactions, responses of HTTP
 * requests or waiting for specific times to pass by.
 *
 * ![](skipUntil.png)
 *
 * Internally, the `skipUntil` operator subscribes to the passed in `notifier` `ObservableInput` (which gets converted
 * to an Observable) in order to recognize the emission of its first value. When `notifier` emits next, the operator
 * unsubscribes from it and starts emitting the values of the *source* observable until it completes or errors. It
 * will never let the *source* observable emit any values if the `notifier` completes or throws an error without
 * emitting a value before.
 *
 * ## Example
 *
 * In the following example, all emitted values of the interval observable are skipped until the user clicks anywhere
 * within the page
 *
 * ```ts
 * import { interval, fromEvent, skipUntil } from 'rxjs';
 *
 * const intervalObservable = interval(1000);
 * const click = fromEvent(document, 'click');
 *
 * const emitAfterClick = intervalObservable.pipe(
 *   skipUntil(click)
 * );
 * // clicked at 4.6s. output: 5...6...7...8........ or
 * // clicked at 7.3s. output: 8...9...10..11.......
 * emitAfterClick.subscribe(value => console.log(value));
 * ```
 *
 * @see {@link last}
 * @see {@link skip}
 * @see {@link skipWhile}
 * @see {@link skipLast}
 *
 * @param notifier An `ObservableInput` that has to emit an item before the source Observable elements begin to
 * be mirrored by the resulting Observable.
 * @return A function that returns an Observable that skips items from the
 * source Observable until the `notifier` Observable emits an item, then emits the
 * remaining items.
 */
export function skipUntil<T>(notifier: ObservableInput<any>): MonoTypeOperatorFunction<T> {
  return (source) =>
    new Observable((destination) => {
      let taking = false;

      const skipSubscriber = operate({
        destination,
        next: () => {
          skipSubscriber?.unsubscribe();
          taking = true;
        },
        complete: noop,
      });

      from(notifier).subscribe(skipSubscriber);

      source.subscribe(operate({ destination, next: (value) => taking && destination.next(value) }));
    });
}
